Reports: AC6
44177-AC6 Structure and Dynamics of Highly Excited Vibrational States of Complexes of Water and Hydrogen Fluoride
A high-resolution study of the complex of carbonyl sulfide (OCS) and hydrogen
(ortho H2 J= 1, para H2 J=0: ortho D2 J= 0, para D2 J=1: and HD J=0) is completed and published. The work is summarized in the publication
Rotational spectra of the van der Waals complexes of molecular hydrogen and OCS
Zhenhong Yu, Kelly J. Higgins, William Klemperer, Michael C. McCarthy, Patrick Thaddeus, Kristine Liao, and Wolfgang Jäger Journal of Chemical Physics 127, 054305 (2007). The spectral resolution is of the order of 10kHz thus the hyperfine structure present in the complexes involving ortho H2, para D2, and HD is well resolved and analyzed. There are few, if any, other complexes of hydrogen that have been studied at this level of resolution.
The analysis of the ortho H2‑ OCS rotational spectrum to determine or estimate the low-lying vibrational levels is close to completion. A high-level electron structure calculation aids this work. These results will soon be complete and submitted for publication. An interesting computational paper by Timothy R. Phillips and Sheldon Green "Excitation of Interstellar Water by ortho- and para Hydrogen " Astrophysics and Space Science 224, 537-538 (1995) estimates the rate of collisional excitation of H2O low lying rotational levels J=1(01) ‑> J=1(10). They find that the rate of excitation by ortho-H2 is ten times faster than by para-H2. This is in good accord with observations of the relative binding strength of the ortho and para H2 form in complexes of hydrogen.
We are calculating accurate potentional energy surfaces for possibly important complexes of H2 and HCO+. HCO+ is the most abundant molecular ion in the interstellar medium. It is well observed in molecular clouds .The question we pose and examine is whether it may lead to the gas phase synthesis of formaldehyde, H2CO. The possible routes for this are either through direct radiative association of HCO+ with H2 forming the complex H2 —HCO+ followed by the displacement reaction H2 —HCO+ +H = H2CO+
+ H2 or by the more direct path H + HCO+ = H2CO+ + photon. The abundance of atomic hydrogen in molecular clouds is high namely [H] = 10-3 H2. Production of neutral formaldehyde by from the ion will occur by charge exchange with low ionization potential species such as NH3, H2S and other sulfides. We note that although HCO+ is well observed in many regions both within the galaxy and in external galaxies, the rotational spectrum of H2CO+ has not yet been observed in the laboratory. Thus the question of its abundance in the galaxy cannot be presently answered It is known that both HCO+ and H2CO+ are stable in an H2 atmosphere [J.Liu and S.L.Anderson "Reation of formaldehyde cation with molecular hydrogen:Effects of collision energy and H2CO+ vibrations" J. Chem. Phys. 120, 8528-8536] The complex H2‑ HCO+ has been well studied by infrared spectroscopy, however, the uncertainty in rotational constants are too large to permit an effective radio-astronomical search for this species in large molecular clouds.
Our interest in these species lies in the great abundance of H2 in molecular clouds. Of the potential complexes with hydrogen these ionic species will have the largest binding energy as well as the greatest collision frequency. Thus they may be important reaction intermediates. The formaldehyde species appear to be a possible reactants in the formation of interesting more complex species which have been observed in the galaxy. Methyl formate, acetic acid and glycolaldehyde have been in several spatial regions. These species are all isomers of the formaldehyde dimer: thus we are exploring synthesis schemes involving the formation of formaldehyde dimer ion followed by charge neutralization . Whether or not this is a feasible synthetic scheme can likely be deduced theoretically
